A Printed Circuit Board Design Project For A Switching Power Converter

A printed circuit board design project is presented through the development and testing of a dcto-dc switching power converter for pulse load applications. Electrical design of power converter integrates the knowledge students have gained in previous courses such as circuit analysis, electronics, electrical machines, control systems, semiconductor devices, and thermal analysis. Use of industry-standard electronic design automation tools was emphasized in the prototype development process including schematic entry and printed circuit board layout. In order to achieve a high density design, use of surface mount components was emphasized over through-hole components. Populating the printed circuit board with surface mount components was carried out through an in-house solder reflow process using a standard toaster oven. The concepts of testing methodologies for high power circuit boards and optimizing the feedback control loop were introduced as part of this project experience. Introduction The design, printed circuit board (PCB) layout, circuit card assembly, and testing of a transformer-based 360 W dc-to-dc switching power converter based on pulse-width modulation technique and voltage/current feedback is presented in this paper. The input to the power converter varies between 24 and 32 V while the output is regulated at 30 V for operation from no-load (0 W) to full-load (360 W). The project required custom magnetics design and stability analysis for the feedback control loop. The schematic entry and board layout were carried out using Multisim and Ultiboard software, respectively. A four-layer board was designed with two layers primarily dedicated to ground plane. The Gerber files were uploaded to a board manufacturers’ server in order to get the PCB manufactured. The board was populated with surface mount parts using an in-house solder reflow process before hand-soldering the through-hole parts. The populated board was then tested thoroughly including optimization of the control-loop to meet the load transient specifications. The loop gain and phase plots were obtained using a frequency response analyzer. Changes incorporated to the power converter design to improve its overall performance are also discussed herein. This design project provided an opportunity to a senior-level undergraduate student to incorporate and integrate knowledge gained in various EET courses by designing a power converter utilizing modern electronic design automation (EDA) and testing tools via a semesterlong credit-bearing independent research course. P ge 1.96.3 Power converter specifications Input voltage: 22 – 32 VDC Output voltage: 30 VDC ± 5% Output voltage ripple: 1% (peak-to-peak) Output load: 0 A (no-load) to 12 A (full load) Isolation: Not required Undervoltage lockout: ON @ 21 V and OFF @ 19 V Switching frequency: 200 kHz Full-load efficiency: ‡ 85% Protection scheme: Cycle-by-cycle current limit Power converter topology Based on the input and output voltage and full-load power (360 W) specifications, a transformer isolated forward converter 1 topology was selected for the design. To protect the converter from excessive current draw due to under voltage conditions, an undervoltage lockout (UVLO) circuit was implemented that turns on the converter when input voltage exceeds 21 V and turns it off when it is under 19 V. Figure 1 shows the startup regulator and the associated UVLO circuit employing op-amp based hysteresis circuit. The startup regulator charges the bias supply capacitors with a maximum current of about 100 mA with a set maximum voltage of 14 VDC. The timeout period for the startup circuit is programmed through R12 and C4.